Make python code compatible with urllib3 v2.6.0+ (#22520)

openapi-generator still uses methods that have been removed from urllib3
v2.6.0. The solution is as described in urllib3's changelog:

> Removed the HTTPResponse.getheaders() method in favor of
> HTTPResponse.headers. Removed the HTTPResponse.getheader(name,
> default) method in favor of HTTPResponse.headers.get(name, default).
> (#3622)

See https://urllib3.readthedocs.io/en/latest/changelog.html

Close #22514

samples/client/echo_api/python/openapi_client/rest.py

@@ -49,12 +49,17 @@ class RESTResponse(io.IOBase):
             self.data = self.response.data
         return self.data
 
+    @property
+    def headers(self):
+        """Returns a dictionary of response headers."""
+        return self.response.headers
+
     def getheaders(self):
-        """Returns a dictionary of the response headers."""
+        """Returns a dictionary of the response headers; use ``headers`` instead."""
         return self.response.headers
 
     def getheader(self, name, default=None):
-        """Returns a given response header."""
+        """Returns a given response header; use ``headers.get()`` instead."""
         return self.response.headers.get(name, default)